unread-byte

Arguments:
stream

This function is similar to unread-char, but it backs up to the previous
octet rather than character. It is also similar in that
if unread-byte is called
immediately after a "back-up" function (i.e. peek-char/peek-byte or unread-char/unread-byte) it may fail. However, it need
not fail; if the stream being operated on is not currently positioned
at the beginning of its buffer, even after a backup function has just
been called, then unread-byte may still succeed. There is
currently no portable way to know whether an unread-byte will work, unless the last
operation on the stream was a read operation other than a back-up
function.